This invention concerns information display instruments such as for instance watches, hand held calculators, instrument panels in vehicles and more particularly the display itself.
The present tendency in this field is toward the adoption of passive displays, in particular liquid crystals. The passive character thereof comes from the fact that such displays do not emit light and they are thus invisible and so non readable in darkness. It is thus necessary to arrange for illuminating means for the display such as an incandescent lamp, an electroluminescent diode or a phosphorescent material activated by tritium.
In the case of small instruments such as watches it is essential to economize the energy and thus to obtain the maximum efficiency in the illumination of the display. Different systems have been proposed in order to guide the light emitted by a source through utilisation of the phenomena of reflection and refraction and in order to diffuse the light at the level of the surface placed under the display according to an appropriate visual angle for the user. These guides generally are basically formed by a thin plate of which the major surface is provided with a structure intended to diffuse light. The surface structures as presently known may be classed in two distinct groups each having its own particular disadvantages.
The first group includes random structures obtained by procedures themselves of a random nature such as for instance abrasion or chemical etching of the surface. They generally provide a fine texture of which the grain is not visible to the naked eye. However they have the major disadvantage that all small defects as for instance dust grains which may be present in the optical system are clearly visible by the observer which is not only rather annoying for the reading but spoils the esthetic effect. An example of such a surface structure is described in the Swiss Pat. No. 589 306.
The second group includes structures having a periodic or regular character obtained by repetitive procedures such as for instance mechanical machining. Such structures generally provide a coarse but regular texture which is perfectly visible to the naked eye. They have not the disadvantage shown up earlier in respect of the first group but present a further disadvantage which is that of not diffusing light coming from the guide except in certain particular directions with opening angles very restrictive and insufficient concerning the necessary visual angle for easy reading.